In the manufacture of UO.sub.2 pellets for use in nuclear reactors, high density pellets are desirable to maximize the amount of fuel per unit volume, but due to the formation of fission products in the fuel some porosity is also desirable in order to contain these fission products. A compromise of these opposing considerations requires that the UO.sub.2 fuel pellets have an optimum density of about 95%.
The UO.sub.2 fuel pellets are prepared by sintering pressed UO.sub.2 powder. If the UO.sub.2 powder is of poor quality, the pellets will sinter to less than the optimum density. In order to achieve higher densities, various dopants may be added to the UO.sub.2 powder prior to sintering. While the addition of the dopants may enable the powder to sinter to the optimum density, they are not normally added during commercial fuel fabrication. Without the addition of sintering aids, a fuel pellet made from poorly sinterable powder results in grains of small size. A small grain size generally is equivalent to a fine porosity which readily disappears in use in the reactor, permitting the escape of fission gases during operation. Second, while the pellets have the optimum density initially, they are often not stable in the reactor and gradually become denser when subjected to the heat of the reactor. In pressurized reactors where the fuel rods themselves are not pressurized, this can result in the collapse of the fuel rod, while gaps can be formed as a result of pellet densification in all fuel rods.
A different sort of problem arises when highly sinterable UO.sub.2 powders are used to prepare the fuel pellets. These powders, which result if the particle size of the UO.sub.2 is very fine, sinter to a density which is above the optimum density. If the sintering process is stopped when the optimum density is reached, the fuel pellets will simply continue to densify in the reactor. Thus, until now, highly sinterable UO.sub.2 powders could not be used without otherwise reducing the sinterability by introduction of large quantities of scrap or oxidized material ("add-back") to reduce the density which also results in an increase in the quantity of fine porosity.